Spear guns are devices for propelling spears and are typically used under water as a weapon or for hunting purposes. Known spear guns use various mechanisms for propelling a spear, one of which is a pneumatic system in which air is compressed to provide the propulsive force. Spear guns which employ a pneumatic system for propelling a spear are commonly referred to as pneumatic spear guns.
A pneumatic spear gun includes a barrel into which the shaft of a spear to be propelled by the spear gun can be received. A piston is slidingly and sealingly received within the barrel. One side of the piston communicates with a supply of air which undergoes compression upon retraction of the piston when the shaft of a spear is introduced into the barrel. The compressed air provides a propulsive force for propelling the piston along the barrel so as to discharge the spear. The air is contained in a reservoir within the body of the spear gun and an inlet is provided for replenishing the air as necessary.
While pneumatic spear guns are satisfactory in operation, they do have a deficiency in that it is not possible to selectively vary the propulsive force which can be applied to the piston.